1. Field of the Invention
The present invention relates to safety apparatus for preventing buckling of pinning rods used with underground mine shaft roof bolting machines.
2. Discussion of Related Art
In U.S. mining practice, passages of rectangular cross-sections are constructed in underground mines. These passages induce failure of the rock near the center of the passage. Mine regulations therefore require the mining company to insert bolts ranging in length from 2 to 8 feet in the mine roof at regular intervals to keep the rock in place. These bolts are installed by a machine which uses a bit and a long extension shank, often called a pinning rod, to first drill a hole in the roof and then to insert a long bolt with an expansion anchor on the upper end. Buckling of the pinning rods during the pinning operation is common because the bolting machines can exert enough thrust to easily bend the pinning rod as it begins to drill into the mine roof. Although the insertion thrust may be manually controlled by the operator, bolting machines have proven to be dangerous and, in numerous cases, machine operators have been seriously injured or even killed when the pinning rods have buckled.
The roof bolting machines are typically hydraulically operated and comprise a torque motor, a pinning rod held for rotation by the torque motor, a hydraulic device for exerting upward thrust on the torque motor to force the pinning rod upwardly into the mine roof, and a device for manually controlling the upward thrust. Many of the roof bolters which are now commercially available have a hydraulically operated boom for raising the torque motor. The boom includes a linkage arrangement for maintaining the pinning rod in a vertical position.
Devices have been suggested for hydraulically controlling the rate of pinning rod infeed so as to increase the drilling rate for softer rock and decrease the drilling rate for hard spots. For example, U.S. Pat. No. 2,320,874 to Lehmann controls the infeed rate and includes a threshold level pressure cutoff which stops the infeed when the pressure in the feed cylinders builds up beyond a predetermined level.
Another type of automatic feed pressure regulator is shown in U.S. Pat. No. 2,322,741 to Osgood. In this device, the feed control is automatically responsive to variations in the resistance to drill bit penetration.
U.S. Pat. Nos. 4,328,872; 4,371,040; and 4,478,290 to Orthwein disclose various mechanisms for preventing buckling of the pinning rod.